1. Field of the Invention
The present invention relates to an optical recording medium which can record and reproduce information in high density and a reproducing method therefor.
2. Description of the Background Art
In relation to an optical disk unit and an optical medium, the so-called MSR (magnetically induced super resolution) system has recently been watched with interest as a super resolution means for recording/reproducing information in/from regions smaller than a recording spot which is defined by the diffraction limit of light. Such an MSR system is disclosed in Technical Digest of Optical Data Storage Topical Meeting, 1991, Vol. 5, pp. 112 to 115 (Lecture No. TUB-3) and pp. 116 to 119 (Lecture No. TUB-4), for example. This system is characterized in that a plurality of magneto-optical recording films are employed with provision of a masking layer for masking peripheral information other than that for an information recording layer. Recorded information (recording marks) is transferred to the masking layer, whose temperature is increased by irradiation, by means of a relatively intense reproducing beam, whereby influences, caused by adjacent tracks and recording marks, in the linear density direction, are suppressed even if the recording density is increased, and therefore the optical resolution is improved.
On the other hand, Japanese Patent Laying-Open No. 5-225611 (1993), for example, discloses an optical recording medium which is provided with a masking layer including a light absorption center causing a nonlinear light absorption phenomenon such as a saturable absorption property, as an optical recording medium for attaining a similar super-resolution effect. Further, Japanese Patent Laying-Open No. 5-242524 (1993) discloses a recording/reproducing method similarly utilizing a nonlinear optical phenomenon, employing a spiro-selenazolino-benzopyran which exhibits inverse photochromism. In addition, Japanese Patent Laying-Open No. 5-266478 (1993) proposes a method employing a masking layer which is generally non-transmittable with respect to a reproducing beam, but exhibits partial transmission only in its central portion upon irradiation with a reproducing beam, which is controlled to exceed prescribed intensity on the central portion and again returns to the opaque state after passage of the reproducing beam. This reference discloses using an indoline spiropyran which exhibits inverse photochromism as a material for the masking layer.
In the conventional method employing the MSR system, however, the recording medium is disadvantageously limited to a magneto-optical recording medium.
Further, the method employing the nonlinear light absorption phenomenon generally requires extremely high light intensity, and hence information which is already recorded in the recording layer may be destroyed by heat generated when new or further information is applied.
In the super-resolution system utilizing inverse photochromism, in addition, reproduction is carried out by scanning the recording layer with a laser beam spot for decoloring the masking layer by a photochromic reaction. However, the masking layer thereafter naturally returns to a colored state due to a thermal reaction, and hence it is difficult to attain compatibility with a medium having no masking layer, i.e., employing no super-resolution optical recording technique. When the beam is impinged on the medium in reproduction, further, a coloring reaction is also caused by a thermal reaction with progress of decoloration by a photochromic reaction, and hence transmittance of the masking layer may be insufficiently improved.